This application is a 35 USC 371 National Phase Entry Application from PCT/JP99/05540, filed Oct. 7, 1999, and designating the U.S.
The present invention relates to a gold plating solution free from sulfite ions, has good stability and is capable of being used for extended periods, and also to a process for gold plating using the gold plating solution.
Gold plating has been traditionally used for decorative purposes including decoration of tableware, and is now widely used in the electronics industries due to the good electrical properties of gold.
Most conventional gold plating solutions contain toxic gold potassium cyanate (potassium tetracyanoaurate). Recently, however, non-cyanide (i.e., cyanide-free) gold plating solutions have been in increasing demand in view of problems resulting from the use of a cyanide bath, such as concerns about safety at work sites, wastewater treatment, and corrosion of resists or the like used in semiconductor elements. Accordingly, various types of non-cyanide gold plating solutions have been proposed.
For example, a type of non-cyanide gold plating solution has been reported in the J. Am. Chem. Soc. 1951, vol. 73, p. 4722, which contains bis(1,2-ethanediamine) gold trichloride as a gold compound. Bis(1,2-ethanediamine) gold trichloride is widely known to be produced by a reaction of chloroauric acid with ethylenediamine (monohydrate) in a solvent (diethyl ether) at ambient temperature. The present inventors has proposed a novel process for producing bis(1,2-ethanediamine) gold trichloride chloride produced by the process to form a gold-plated layer with a good appearance. In such a plating solution or process, however, control Of e hardness, purity and state of the gold crystals deposited by the plating has yet been impossible.
On the other hand, widely available non-cyanide gold plating solutions often contain Na3Au(SO3)2 as a gold salt. In a gold plating solution bath containing Na3Au(SO3)2, however, sulfite ions in the gold plating solution are highly unstable and, therefore, may be readily oxidized by oxygen generated from an anode or that present in the atmosphere, causing a spontaneous reduction in the concentration of sulfite ions in the gold plating solution. As a result, the stability of gold complexes in the gold plating solution is decreased, leading to inconvenience such as changes in the physical properties of the electrolytically deposited materials or decomposition of the plating solution.